1. Field of the Invention:
This invention relates to electrical wiring distributing floor structures, and more particularly to improved means for fireproofing an underfloor access chamber of such floor structures.
2. Description of the Prior Art:
Floor structures particularly adapted to distribute electrical wiring are known which comprise, for example, a metal cellular unit providing enclosed wire carrying cells, a covering layer of concrete, and one or more inserts positioned between the metal cellular unit and the covering layer of concrete. Each insert provides an access chamber communicating with at least one of the cells of the cellular unit. Access to the chamber is achieved by removing a covering portion of the concrete and installing capping means. The capping means provides ready access to the chamber for making connections to the individual electrical services available therein at one location in the floor structure.
Such floor structures must be capable of containing a fire burning, for example, in the space below the floor structure for a period of time sufficient to permit the building occupants to move safely out of the building. The fire containment time--commonly referred to as the "fire rating"--is expressed as "2-hour," "1/2-hour," etc.
Useful "fire ratings" are achieved by the addition of external and internal fireproofing to the floor structure. The external fireproofing--provided at the lower face of the floor structures--may comprise field-sprayed material, gypsum board or other membranes such as fire rated suspended ceiling. Internal fireproofing--installed within the chamber--may comprise plural components of intumescent material, rigid glass fiberboard, standard gypsum wallboard and gypsum blocks, etc. Examples of inserts incorporating intumescent material will be found in U.S. Pat. Nos. 3,864,883 (McMARLIN) and 3,932,696 (FORK et al.). An example of a floor structure incorporating external and internal fireproofing will be found in copending application Ser. No. 901,063 (GRAY et al.) filed Apr. 28, 1978 and assigned to the assignee of the present invention.
Under fire conditions, heat is transmitted through the floor structure from the fire-exposed face to the unexposed face. Heat transmission occurs by conduction, by convection and by radiation. The external and internal fireproofing and the inherent fire-resistant properties of the floor structure components cooperate to retard heat transmission thereby extending the time period--the "fire rating"--during which the unexposed face of the floor structure attains a critical elevated temperature. Once the exposed face attains the critical elevated temperature, the floor structure is no longer considered capable of performing its fire-containment function.
The intumescent materials disclosed by McMARLIN and FORK et al., when activated, foam and expand to substantially entirely fill the chamber. However, activation occurs only after the material is subjected to an elevated temperature, e.g., temperatures in excess of 250.degree. F. Up to activation, the capping means at the unexposed surface of the floor structure are continuously subjected to the ever increasing temperature.
The arrangement of internal insulating components disclosed by GRAY et al. provide the floor structure with useful "fire ratings." However, the multiple insulating components add significantly to the cost of capping means and reduce the space available within the chamber for making electrical connections.